The cosmic ultraviolet baryon survey (CUBS) - III. Physical properties and elemental abundances of Lyman-limit systems at z < 1

Abstract

We present a systematic investigation of physical conditions and elemental abundances in four optically thick Lyman-limit systems (LLSs) at z = 0.36-0.6 discovered within the cosmic ultraviolet baryon survey (CUBS). Because intervening LLSs at z < 1 suppress far-UV (ultraviolet) light from background QSOs, an unbiased search of these absorbers requires a near-UV-selected QSO sample, as achieved by CUBS. CUBS LLSs exhibit multicomponent kinematic structure and a complex mix of multiphase gas, with associated metal transitions from multiple ionization states such as CII, CIII, NIII, MgII, SIII, SIIII, OII, OIII, OvI, and FeII absorption that span several hundred km s(-1) in line-of-sight velocity. Specifically, higher column density components (logN(HI)/cm(-2) greater than or similar to 16) in all four absorbers comprise dynamically cool gas with K and modest non-thermal broadening of km s(-1). The high quality of the QSO absorption spectra allows us to infer the physical conditions of the gas, using a detailed ionization modelling that takes into account the resolved component structures of Hi and metal transitions. The range of inferred gas densities indicates that these absorbers consist of spatially compact clouds with a median line-of-sight thickness of pc. While obtaining robust metallicity constraints for the low density, highly ionized phase remains challenging due to the uncertain , we demonstrate that the cool-phase gas in LLSs has a median metallicity of , with a 16-84 percentile range of [alpha/H] = (-1.3, -0.1). Furthermore, the wide range of inferred elemental abundance ratios ([C/alpha], [N/alpha], and [Fe/alpha]) indicate a diversity of chemical enrichment histories. Combining the absorption data with deep galaxy survey data characterizing the galaxy environment of these absorbers, we discuss the physical connection between star-forming regions in galaxies and diffuse gas associated with optically thick absorption systems in the z < 1 circumgalactic medium.